Doped graphene films with reduced sheet resistance

ABSTRACT

Techniques for increasing conductivity of graphene films by chemical doping are provided. In one aspect, a method for increasing conductivity of a graphene film includes the following steps. The graphene film is formed from one or more graphene sheets. The graphene sheets are exposed to a solution having a one-electron oxidant configured to dope the graphene sheets to increase a conductivity thereof, thereby increasing the overall conductivity of the film. The graphene film can be formed prior to the graphene sheets being exposed to the one-electron oxidant solution. Alternatively, the graphene sheets can be exposed to the one-electron oxidant solution prior to the graphene film being formed. A method of fabricating a transparent electrode on a photovoltaic device from a graphene film is also provided.

FIELD OF THE INVENTION

The present invention relates to graphene, and more particularly, totechniques for increasing the conductivity of graphene films by chemicaldoping.

BACKGROUND OF THE INVENTION

A conductive transparent electrode is an integral component of aphotovoltaic cell. Indium tin oxide (ITO) is currently the most commonlyused transparent electrode material. Although ITO offers excellentoptical and electrical properties, the fabrication of an ITO electrodeinvolves costly vacuum deposition techniques. ITO (and other metaloxides) also suffer from being brittle, and thus are incompatible withflexible substrates. Further, with the increasing costs of mined metals,ITO is becoming a less economically viable solution for large scalephotovoltaic cell production.

Graphene is considered a leading candidate to replace ITO as thetransparent electrode material in photovoltaic devices since it can besolution processed, which may significantly drive down the cost ofphotovoltaic device fabrication and allow for compatibility withvirtually any substrates. As-prepared graphene sheets typically have asheet resistance of from about 250 ohms per square (ohm/sq) to about4,000 ohm/sq, depending on the fabrication process. To be useful as atransparent electrode material in photovoltaic devices, the sheetresistance of the as-prepared transparent graphene films needs to bereduced.

Two approaches can be pursued to reduce the sheet resistance ofgraphene: stacking of several graphene films on top of each other and/orchemical doping. Stacking of graphene films essentially adds additionalchannels for charge transport. However, this approach simultaneouslyreduces the transparency of the system. See, for example, Li et al.,“Transfer of Large-Area Graphene Films for High-Performance TransparentConductive Electrodes,” Nano Letters, vol. 9, no. 12, pgs. 4359-4363(2009). In addition, since the electronic properties are essentiallypreserved in the stacked graphene films (SGF), alternative solutionssuch as doping must also be considered. See, for example, Jung et al.,“Charge Transfer Chemical Doping of Few Layer Graphenes: ChargeDistribution and Band Gap Formation,” Nano Letters, vol. 9, no. 12, pgs.4133-4137 (2009) (hereinafter “Jung”).

Graphene is classified as a semi-metal or zero-gap semiconductor wherethe density of states vanishes at the Dirac point. Undoped graphene hasa low carrier density, and thus high sheet resistance, due to itsvanishing density of states at the Dirac point. Due to unintentionaldopants the Fermi level most certainly will not reside at the Diracpoint of chemical vapor deposition (CVD)-grown graphene films exposed toair, yet chemical doping should still inject sufficient carriers toreduce the resistance of the film. This can be accomplished by injectingcharges that result in a shift in the graphene Fermi level withoutinterrupting the conjugated network. See, for example, Jung. Doping ofSGF shifts the Fermi level further away from the Dirac point leading toa large increase in the conductivity. See, for example, Voggu et al.,“Effects of Charge Transfer Interaction of Graphene with Electron Donorand Acceptor Molecules Examined Using Raman Spectroscopy and CognateTechniques,” J. Phys. Condens. Matter 20, pg. 472204 (2008), Lu et al.,“Tuning the Electronic Structure of Graphene by an Organic Molecule,” J.Phys. Chem. B, 113, 2-5 (2009) and Eberlein et al., “Doping of Graphene:Density Functional Calculations of Charge Transfer Between GaAs andCarbon Nanostructures,” Phys. Rev. B, 78, 045403-045408 (2008). Stackingof graphene sheets leads to a reduction in transparency of the graphenefilms, which is detrimental for transparent electrodes. Current graphenedoping techniques provide dopants that are not stable in time.

Therefore, improved techniques for reducing the sheet resistance oftransparent graphene films would be desirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for increasing conductivity ofgraphene films by chemical doping. In one aspect of the invention, amethod for increasing conductivity of a graphene film includes thefollowing steps. The graphene film is formed from one or more graphenesheets. The graphene sheets are exposed to a solution having aone-electron oxidant configured to dope the graphene sheets to increasea conductivity thereof, thereby increasing the overall conductivity ofthe film. The graphene film can be formed prior to the graphene sheetsbeing exposed to the one-electron oxidant solution. Alternatively, thegraphene sheets can be exposed to the one-electron oxidant solutionprior to the graphene film being formed. A method of fabricating atransparent electrode on a photovoltaic device from a graphene film isalso provided.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for increasingconductivity of a transparent graphene film according to an embodimentof the present invention;

FIG. 2A is a diagram illustrating a photovoltaic device according to anembodiment of the present invention; and

FIG. 2B is a diagram illustrating a graphene film which will serve as atransparent electrode having been formed on a surface of thephotovoltaic device according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques for using solution chemistry to heavilydope graphene, thereby reducing the sheet resistance in transparentgraphene films by a factor of from about two to about four. FIG. 1, forexample, is a diagram illustrating exemplary methodology 100 forincreasing conductivity of a transparent graphene film.

In step 102, graphene sheets are prepared using a conventional process.By way of example only, chemical vapor deposition (CVD) onto a metal(i.e., foil) substrate can be used to form the graphene sheets. See, forexample, Li et al., “Large-Area Synthesis of High-Quality and UniformGraphene Films on Copper Foils,” Science, 324, pgs. 1312-1314 (2009)(hereinafter “Li”) and Kim et al., “Large-Scale Pattern Growth ofGraphene Films for Stretchable Transparent Electrodes,” Nature, vol.457, pgs. 706-710 (2009) (hereinafter “Kim”), the contents of each ofwhich are incorporated by reference herein. Chemical exfoliation mayalso be used to form the graphene sheets. These techniques are known tothose of skill in the art and thus are not described further herein. Ashighlighted above, the as-prepared graphene sheets typically have asheet resistance of from about 250 ohms per square (ohm/sq) to about4,000 ohm/sq, depending on the fabrication process. As known by those ofskill in the art, sheet resistance and conductivity are inverselyrelated to one another, i.e., as sheet resistance decreases conductivityincreases, and vice-a-versa. Advantageously, the present teachingsprovide techniques for reducing the sheet resistance/increasing theconductivity of films formed from these graphene sheets (see below).

In step 104, the graphene sheets are formed into a film. According to anexemplary embodiment, the film is formed by depositing the graphenesheets onto a given substrate, e.g., a photovoltaic device, usingconventional lift-off techniques (see, for example, Li and Kim). Ingeneral, the sheets are deposited one on top of another to form thefilm. Thus, by way of example only, the graphene film can comprise astack of five graphene sheets (also called layers). The term “substrate”is used to generally refer to any suitable substrate on which one wouldwant to deposit a graphene film. By way of example only, the substratecan be a photovoltaic device, on which the carbon nanotube film isdeposited as a transparent electrode material.

In step 106, a solution is prepared containing a one-electron oxidant ina solvent. According to an exemplary embodiment, the one-electronoxidant is triethyloxonium hexachloroantimonate. Suitable solventsinclude, but are not limited to, one or more of methylene chloride,dimethylformamide (DMF), chloroform and acetone. A typical preparationinvolves adding 10 milligrams (mg) of the one-electron oxidant to 10milliliters (ml) of solvent. The solution is stirred or sonicated untilthe one-electron oxidant completely dissolves into the solution.

In step 108, the transparent graphene film is exposed to theone-electron oxidant solution. According to an exemplary embodiment, thefilm is soaked in the one-electron oxidant solution for a duration of atleast about 10 minutes, e.g., for a duration of about 30 minutes. By wayof example only, if the film is being used as a transparent electrodematerial for a photovoltaic device, then the film can first be depositedon the device and the device with the film exposed to (e.g., soaked in)the one-electron oxidant solution. After the film is exposed and soakedfor a proper length of time, it is simply removed from the solution andrinsed with an appropriate solvent, such as acetone. Exposing the filmto the one-electron oxidant solution serves to dope the graphene.

Exposing the graphene film to the one-electron oxidant solution shiftsthe graphene Fermi level further away from the Dirac point, leading to alarge increase in the conductivity and reduction of the sheet resistancewithout interrupting the conjugated network.

The dopant reduces the sheet resistance by at least a factor of two,i.e., by a factor of from about two to about four. By way of exampleonly, in one exemplary implementation of the present techniques, afive-sheet (layer) thick graphene film on a quartz substrate with atransparency of 83 percent (%) at 550 nanometers (nm) and a sheetresistance of 460 ohm/sq was dipped in a solution of triethyloxoniumhexacloroantimonate. Due to doping of the graphene, the sheet resistancedropped to 120 ohm/sq, while the transparancy stayed the same.

Further, advantageously, the doped film has enhanced stability ascompared with other doping methods. For example, doped films preparedaccording to the present techniques remain stable even after severalmonths. In fact, the process should last indefinitely especially if,e.g., the photovoltaic device containing the present doped film isencapsulated in some sort of polymer. The present method is more stablebecause the metal salts form a charge transfer complex with the graphenethat is difficult to reverse.

Alternatively, the graphene sheets can be doped prior to forming theminto the film, and achieve the same results. Namely, in step 110, theone-electron oxidant solution, e.g., triethyloxoniumhexachloroantimonate in methylene chloride, DMF, chloroform and/oracetone, is prepared. The process for preparing the one-electron oxidantsolution was described in detail above.

In step 112, the graphene sheets are exposed to the one-electron oxidantsolution. According to an exemplary embodiment, the sheets are soaked inthe one-electron oxidant solution for a duration of at least about 10minutes, e.g., for a duration of about 30 minutes. As described above,exposing the sheets to the one-electron oxidant solution serves to dopethe graphene. In step 114, the graphene sheets (now doped) are formedinto a film.

Advantageously, if chemical exfoliation is used to produce the graphenesheets (see above), the present techniques are completely solutionbased, which has enormous cost advantages in photovoltaic fabrication.Namely, the raw materials used in the present process are cheaper(carbon versus indium (see above)), the process is entirely fromsolution and there is no need for expensive vacuum deposition techniques(see above). Further, the doping procedure is independent of the methodof graphene film deposition being used.

FIGS. 2A and 2B are diagrams illustrating an exemplary methodology forfabricating a transparent electrode on a photovoltaic device from atransparent graphene film. A generic photovoltaic device is shown inFIG. 2A. The photovoltaic device includes a bottom electrode 202, afirst photoactive layer 204 and a second photoactive layer 206. By wayof example only, the first and second photoactive layers can be doped soas to have opposite polarities from one another, e.g., one is doped witha p-type dopant and the other is doped with an n-type dopant. In thisexample, a p-n junction would be formed between the two photoactivelayers. Such a generic photovoltaic device would be apparent to one ofskill in the art and thus is not described further herein. Further, aswould be apparent to one of skill in the art, there are a multitude ofdifferent photovoltaic device configurations possible, and theconfiguration shown in FIG. 2A is provided merely to illustrate thepresent techniques for fabricating a transparent electrode on thephotovoltaic device from a graphene film having increased conductivity.

As shown in FIG. 2B, graphene film 208 which will serve as thetransparent electrode is formed on a surface of the photovoltaic device,in this example on a surface of second photoactive layer 206. Asdescribed above, the conductivity of the graphene film can be increasedby exposing the graphene to a solution containing a one-electron oxidant(e.g., triethyloxonium hexachloroantimonate) either by exposing theindividual sheets (i.e., prior to forming the film) or as a film, todope the graphene. By way of example only, the photovoltaic device withthe film formed thereon can be exposed to (e.g., soaked in) theone-electron oxidant solution.

As highlighted above, the graphene film 208 can be formed on the surfaceof the photovoltaic device in a number of different ways. By way ofexample only, graphene film 208 can be formed by depositing the graphenesheets onto the surface of the photovoltaic device using wet chemistry.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

1. A method for increasing conductivity of a graphene film, comprisingthe steps of: forming the graphene film from one or more graphenesheets; and exposing the graphene sheets to a solution comprising aone-electron oxidant configured to dope the graphene sheets via chargetransfer to increase a conductivity thereof while leaving a conjugatednetwork of the graphene sheets uninterrupted, thereby increasing theoverall conductivity of the film.
 2. The method of claim 1, wherein thestep of forming the graphene film is performed prior to the step ofexposing the graphene sheets to the one-electron oxidant solution. 3.The method of claim 1, wherein the step of exposing the graphene sheetsto the one-electron oxidant solution is performed prior to the step offorming the graphene film.
 4. The method of claim 1, further comprisingthe step of: preparing the solution of the one-electron oxidant in asolvent.
 5. The method of claim 4, further comprising the step of:stirring the solution until the one-electron oxidant completelydissolves.
 6. The method of claim 4, wherein the one-electron oxidantcomprises triethyloxonium hexachloroantimonate.
 7. The method of claim4, wherein the solvent comprises one or more of methylene chloride,dimethylformamide, chloroform and acetone.
 8. The method of claim 1,wherein the step of exposing the graphene sheets to the one-electronoxidant in solution comprises the step of: soaking the sheets in thesolution.
 9. The method of claim 8, wherein the sheets are soaked in thesolution for a duration of about 30 minutes.
 10. The method of claim 8,further comprising the step of: rinsing the sheets with a solvent afterthe step of soaking the sheets in the solution is performed.
 11. Themethod of claim 10, wherein the solvent comprises acetone.
 12. Themethod of claim 1, further comprising the step of: depositing thegraphene sheets on a substrate to form the graphene film.
 13. The methodof claim 12, wherein the graphene sheets are deposited on the substrateusing lift-off techniques.
 14. The method of claim 12, wherein thesubstrate comprises at least a portion of a photovoltaic device.
 15. Agraphene film having increased conductivity prepared by the method ofclaim
 1. 16. A method of fabricating a transparent electrode on aphotovoltaic device from a graphene film, comprising the steps of:forming the graphene film from one or more graphene sheets; and exposingthe graphene sheets to a solution comprising a one-electron oxidantconfigured to dope the graphene sheets via charge transfer to increase aconductivity thereof while leaving a conjugated network of the graphenesheets uninterrupted, thereby increasing the overall conductivity of thefilm.
 17. The method of claim 16, wherein the step of forming thegraphene film is performed prior to the step of exposing the graphenesheets to the one-electron oxidant solution.
 18. The method of claim 16,wherein the step of exposing the graphene sheets to the one-electronoxidant solution is performed prior to the step of forming the graphenefilm.